Method of carburizing steel and steel alloys, and fused salt bath thereof



United States Patent O- METHOD OF CARBURIZING STEEL AND STEEL ALLOYS, AND FUSED SALT BATH THEREOF No Drawing. Application March 23, 1956 Serial No. 573,331

Claims priority, application Great Britain March 25, 1955 10 Claims. (Cl. 148-155) This invention relates to the heat treatment of steels and alloy steels and to improved fused salt baths and compositions therefor.

It is knownthat fused salt baths of widely different compositions may be employed for the heat treatment of steels and that steel articles immersed in such salt baths will emerge after treatment carburised by the action of the mixture of fused salts, the depth of carbon penetration depending always on the precise conditions under which treatment is carried out. It is common practice to employ for this purpose a fused mixture of cyanides and carbonates of the alkali metals or else such cyanide/- carbonate mixtures in association with one or more of the halides of the alkali metals and in some 'cases the molten contents of the bath are composed of cyanide and halides of the alkali metals which may be in admixture with alkaline earth chlorides and carbonates. In most cases when such molten baths are used for heat treatment purposes it is usual to cover the molten salt mixture with a layer of graphitic carbon.

The particular bath composition desirable for heat treatment of steel articles depends to a large extent upon the degree and depth of carburisation that is required in the treated product. If it is required that the carburised casing on the steel articles should be relatively small, for instance up to 0.020 to 0.025 inch then it is usual to employ fused salt mixtures of the cyanides and carbonates with a halide or mixture of halides of the alkali metals.

When greater depths of the carburised layer, or a better degree of saturation in the carburised case is required, it is usual to employ a salt composition consisting of a major proportion of the chlorides of the alkaline earth metals barium and strontium together with 10%-20% of alkali metal cyanide.

The alkali metal halide/cyanide/carbonate baths are satisfactory, however, for many purposes and they have the advantage that the salt mixtures concerned are soluble in water and the salts therefore may easily be removed from the steel articles treated in the bath. The baths containing barium chloride or strontium chloride with 10% to 20% of alkali metal cyanide suffer from the defect that they leave a deposit on the articles treated which is insoluble in water or at the best only very sparingly soluble; the articles therefore require to be specifically cleaned after treatment in the bath and if salt deposit is not entirely removed rusting is likely to occur.

According to the present invention an improved fused salt bath for the heat treatment of steels and alloy steels comprises more than 50% by weight of an alkali metal halide or mixture of alkali metal halides, an alkali metal cyanide and 0.25% to 5% by weight of a regenerator which is titanium or a mixture thereof with silicon and/ or silica.

The cyanide content of the bath must be less than that of the alkali metal halide and advantageously we use up to 15% and preferably from 1% to by Weight of alkali metal cyanide.

2,846,345 Patented Aug. 5,1958

We prefer to include in the alkali metal halidesaquantity of sodium fluoride amounting to. less than 10% by weight of the weight of the bath.

Titanium is preferably employed as the regenerator. We have found that although titanium dioxide maybe employed as an alternative to titanium less useful results are obtained. For instance in contradistinction totitanium when using titanium dioxide as regenerator intergranular penetration of the surface lay'eryof alloy steels is liable to occur. a

Suitably the regenerator is present in the bath in a form having a large ratio of surface to volume. such as powder. Addition of the regenerator to 'the bath is conveniently effected by enclosing powdered regenerator in a bag and adding to the bath or by mixing the powdered regenerator with a small amount of one or-more of the salt constituents of the bath, for instance, asalt mixture of the same composition as the original fused melt and adding the mixture of powdered regenerator and salt to the bath. 1

Although useful results may be obtained when operating the bath containing up to 5% of the regenerator considerably less than the said 5% may bev employed.

The particular proportion of regenerator to be added suitably per 8 hour day, depending somewhat on the conditions under which the bath is employed. I V

The bath may be operated at any temperature above the fusion point of the salt mixture and as the temperature is increased above this point the rate of carburisation becomes greater. The actual temperature employed is dependent on the composition of the salt bath, but generally we prefer to operate at a temperature in the range 800 C. to 950 C.

One suitable method of operating the bath is firstto make a melt of the alkali metalhalides for example sodium chloride and potassium chloride or a mixture of sodium chloride, potassium chloride and sodium fluoride and maintain the melt at a temperature of 750 C. to 950 C., 1% to 10% by weight of sodium cyanide being added to the bath. Titanium powder amounting to 1% to 2% of the weight of the bath is then dispersed therein by either of the methods previously described. The bath is dredged at regular intervals, preferably each day, and the sludge formed in the process of carburising the steels is removed. At such intervals the bath is made up to the working level by further additions of sodium/potassium chloride mixture'or chloride/fluoride mixture','as the case may be, and by addition of suflicient' sodium cyanide to make good any losses. When operating the bath for an 8-hour day a daily addition of 1% to 2% by weight of powdered titanium may suitably be added. When making up the bath after the addition of titanium it may also be a convenient time to addpowdered graphite to the bath so as to form a layer on the surface and it must be understood that the present invention envisages the presence of a powdered graphite layer in the bath whenever desirable.

The bath compositions of the present invention offer a number of advantages over salt baths which have been employed hitherto for heat treatment purposes. For instance, they confer on the articles treated a depth of carburisation which is superior to that conferred by Th l n percentages are mentioned they are by weight.

' metal'salt baths which comprise cyanides and carbonates and which may also contain alkali metal halides. Also the cyanide usage of our baths is less than that of the said carbonate-containing baths. Further,

i thejsitua'tion which is found'with baths containing alkaline earth'chlorides and alkali metal cyanides the salts adhering to the steel articles treated in the baths of the present inventionmay readily be washed ofi with water omP t ns;

examples illustrate' the' invention. Where v Examplel A fused salt bath of weight 50 lb. containing 10% f'NaCN, "42.5%" KCl, 42.5% NaCl and NaF was madeup and heated to 950 C. Titanium powder amounting'to 1% of the weight of the bath contained in a linen bag was then added to the bath and the surfaceof'the bath was covered with a layer of graphitic carbon'and pieces'of carbon case hardening steel as described'in British Standard Specification 970:1947 under No. EN32 were immersed in the bath for a standard period of 2 hours. The bath was in use 8 hours per day for -90- days, the sludge being removed daily. A mixture containing 35% NaCN, 5% NaF, 28.8% KCI, 28.8%

NaCl and 2.5% S102 was added daily to the bath to maintain asfai' as possible a concentration of 10% NaCN and to make good other salt losses. Titanium powder in amount equal to 1% of the weight of the bath was also "added daily to the bath in the manner described except "on-the 74th day when 3% of titanium powder was added.

The bath was analysed from time to time and after treatment the steel articles 'were allowed to cool slowly and lnfthe courseofithe run it was found that a test piece of analloy. steel containing nickel and chromium as described ,in British Standard Specification No. 8.82

treatedin this bath for 3 hours at 900 C. was free from ,inte'r'granular penetration. The analysis of the alloy steel containing' nickel'and chromium as described in British Standard Specifican'on .No. 8.82 is as follows:

. Percent Carbon' 0.12-0.18 fSilicon" 0.10-0.35 Manganese 0.5 max. Nickel 3.8-4.5

Chromium 1.0-1.4

Example 2 I A fused salt bath was made up and run as in Example 1 but in this case 0.5% powdered titanium and 0.5%

powdered silicon were added daily to the bath and a salt mixture containing 35% NaCN, 5% NaF, 30%" KCl and 30% NaCl was added daily to the bath to maintain as far as possible a concentration of 10% NaCN and to make good other salt losses. The bath was in use for 52 days and was analysed from time to time. After treatment for 2 hours at 950 C. test pieces of carbon casehardening steel as described in British Standard Specification 970:1947. No. EN32 were allowed to cool slowly and examined. The following resultswere obtained: A

Sodium Total 1 Case Eutectold Cyanide Case Depth, Depth, Content, Depth, 0.6% 0.9%

percent inches Carbon, Carbon,

inches inches The analysis of the steel described in British Standard Specification 970: 1947 under N0.EN32 is as follows:

Percent Carbon 0. 10-0. 18 Silicon 0.05-0.35 Manganese 0.6-1.0 Sulphur 0.07 max. Phosphorus; 0.05 max.

What we claim is: p

1. A fused salt bath for the heat treatment of steels and alloy steels which comprises more than 50% by weight of at least one alkali metal halide, an alkali metal cyanide and-0.25% to 5% by weight of titanium.

2. A'fused salt bath as claimed in claim 1 in which the titanium is in a form having a large ratio of surface to volume.

3. A fused salt bath as claimed in claim 1 containing 0.75% to 2% by weight of titanium.

4. A fused salt bath as claimed in claim 1 containing up to 15% by weight of an alkali metal cyanide.

5. A fused salt bath as claimed in claim 4 containing 1% to 10% by weight of an alkali metal cyanide.

6. A fused salt bath as claimed in claim 1 in which the alkali metal halide contains a quantity of sodium fluoride amounting to less than 10% by Weight of the weight of the bath.

7-. A process for heat treating steel and alloy steel halide, an alkali metal cyanide and 0.25% to 5% by weight of titanium.

8. A process as claimed in claim 7 in which the temperature of the bathis in the range of 800 C. to 950 C.

9. A fused salt bath as claimed in claim 1 wherein said titanium is in admixture with a member of the group consisting of silicon and silica. f

10. A fused salt bath as claimed in claim 1 including a mixture of alkali metal halides.

References Cited in the file of this patent UNITED STATES PATENTS 2,063,079 Beck Dec. 8, 1936 2,364,292 -Holt Dec. 5, 1944 2,711,981 7 Waterfall et al June 28, 1955 

7. A PROCESS FOR HEAT TREATING STEEL AND ALLOY STEEL WHICH COMPRISES HEATING THE STEEL IN A FUSED SALT BATH COMPRISING MORE THAN 50% BY WEIGHT OF AN ALKALI METAL HALIDE, AN ALKALI METAL CYANIDE AND 0.25% TO 5% BY WEIGHT OF TITANIUM. 